Extra mass in atoms?

I'm a highschool kid taking chemistry, and I'm confused about how atoms get their listed mass. Take Neon for instance, it's got 10 protons, 10 neutrons, and 10 electroncs, it's normally not an ion or isotopic. So from what my teacher's explained, Neon shold have an Atomic Mass of 10 from the protons, 10 from the neutrons, and around .005 from 10 electroncs. However, on the periodic table of elements, it says Neon has an atomic mass of 20.18. So where do these extra .175 AMU's come from? Has he over-simplified something and Protons or Neutrons really weigh a bit more than exactly 1 AMU, or is there something else about Atoms that gives them more weight than they should have?

genrarly, we ignore the mass of the electrons because they are so insignificantly small, you would need 1835 electrons to equal the mass of 1 proton, and you cant get that much on a single atom becuase you would normally need that many protons to cancel out the charge, and the periodic table does quite reach 1835 elements.